1. Field of the Invention
This invention relates generally to tar acid and more particularly to the reduction of the inherent impurity constituents in tar acids.
2. Prior Practice
Crude tars contain tar acids, tar bases, and neutral oils. These are commercially valuable substances. The crude tar is subjective to distillation to remove these compounds and change the characteristics of the tar. Unfortunately, the boiling point of the various ingredients of the crude tar tend to overlap and therefore the specific ingredients are not readily separated by distillation of the tar. Accordingly, the tar is first subjective to distillation that gives cuts encompassing a wide range of temperatures.
The first distillate fraction is that which distills from tar at temperatures below 230.degree.-240.degree. C. is generally referred to as "tar acids" and is mainly comprised of hydroxy benzenes such as phenols and homologues. This distillate may then be separated either by chemical means or by physical means such as fractional distillation into comparatively pure components, usually as phenol, ortho cresol, meta and para cresol and the six isomers of xylenols. Usually the distillate fraction also include some "tar bases" which are mainly cyclic, nitrogen containing compounds such as pyridine, picoline, lutidine, collidine, aniline, toluidine, xylidine, quinoline, isoquinoline and quinaldine. The distillate fraction may also include some "neutral oil" which is comprised hydrocarbon derivatives of benzene and naphthalene. As may be expected, the composition of a cut depends upon the tar from which the cut is obtained.
The main source of tar acids has heretofore been the tar that is obtained as the by-product of the coking of coal. The tar acids fraction obtained by the distillation of this tar is about 10-20% of the crude coal tar.
Recently a source of tar acids has become available from the Lurgi gasification process. The Lurgi process uses oxygen and steam to gasify brown coal, lignite and non coking sub-bituminous coals in a fixed bed at pressures of 0 to 20 atmospheres and produce a fuel gas. The crude gas leaving the gasifies contains carbonization products such as tar, oil, naphtha, phenols, cyanides, and coal and ash dusts. The gas is cleaned, i.e., these products are removed from the gas before the gas is used as a fuel. The tar that is thus obtained is subjected to distillation in the same manner as in the tar obtained from the production of coke to obtain various distillation cuts.
Tar acids are valuable commercially in the production of numerous items such as resins, plasticizers, and disinfectants. The boiling points of the tar acids, tar bases and neutral oil are such that they cannot be effectively separated by distillation alone. The contamination of the tar acids by the tar bases and neutral oils impair the utility of the tar acids.
The tar acid distillate cut from the by-product tar from the Lurgi process and popularly termed "Lurgi tar acids" has a composition typically comprising 93% tar acids, 5% tar base and 2% neutral oil.
Heretofore, tar acids containing neutral oils and tar bases have usually been extracted with certain selected solvents or combinations of solvents which extracts contained neutral oils leaving a mixture of tar bases and tar acids. The tar bases may be extracted from this mixture with an aqueous solution of a mineral acid, usually aqueous sulfuric acid. Finally, the remaining tar acids are water washed before distilling. The mineral acid extraction must be such that the tar acids are free or nearly free of nitrogenous compounds in order to be salable.